From David Rhodes
At one time I had a six foot tunnel on my [old] line.
To ensure access in the case of derailment or hibernating wild life, the tunnel top was removable in 2' sections.
I cast two tunnel portals in concrete, using an old rubber car mat to form the stone effect.
These were concreted into place and the gap between them had two rows of paving slabs (one either side of the track). Three wooden trays were made [2' long], these sat on top of the paving slabs to form the roof of the tunnel. The trays were filled with soil and plants added to taste, which when they took hold disguised the fact that there were any trays in the vicinity.
The garden was then landscaped around the sides of the tunnel.
Just to add interest to the whole project, the tunnel was built on a length of line which was on a curve, 9' radius.